Most industrial aqueous systems contain alkaline earth metal cations, such as calcium, magnesium, and the like, as well as numerous anions such as bicarbonate, carbonate, sulfate, and the like. When the concentration of the various combinations of cation and anions exceed the solubility of their reaction products, precipitates tend to form until the product solubility concentrations are no longer exceeded. As these reaction products precipitate on the surfaces of the aqueous systems, they form what is known as scale. The precipitation of calcium carbonate is by far the most common form of scale in industrial aqueous systems. This occurs when the ionic product of calcium and carbonate exceeds the solubility of the calcium carbonate and a solid phase of calcium carbonate forms.
The formation of scale in industrial aqueous systems represents a major problem since it reduces heat transfer efficiency on heat exchanger surfaces, increases corrosion problems and reduces flow of the water through the system. The addition of inorganic and, more recently, all organic polyphosphonates to these aqueous systems is known to inhibit scale formation. These compositions are generally added to the system in substoichiometric amounts to the scale forming salt and are known to those in the art as threshold inhibitors. Threshold inhibition describes the phenomenon whereby a sub-stoichiometric amount of a scale inhibitor can stabilize a solution from precipitation. Threshold inhibition generally takes place under conditions where a small amount, e.g. 1 ppm to 100 ppm of an additive, will stabilize the solution which contains many orders of magnitude greater concentration of scale forming salts.
Iron and iron-based metal-containing alloys, such as mild steel, are well-known materials used in constructing the apparatus of aqueous systems. In these systems water circulates, contacts the ferrous-based metal surface, and may be concentrated, such as by evaporation, of a portion of the water from the system. Even though such, metals are readily subject to corrosion in such environments, they are used over other metals due to their strength and availability.
It is known that various materials which are naturally or synthetically occurring in the aqueous systems, especially systems using water derived from natural resources such as seawater, rivers, lake and the like, attack ferrous-based metals. The term "ferrous-based metals", as used herein refers to any ferrous-containing metals. Typical devices in which the ferrous-based metal parts are subject to corrosion include evaporators, single and multi-pass heat exchangers, cooling towers, and associated equipment and the like. As the system water passes through or over the device, a portion of the system water evaporates causing a concentration of the dissolved materials contained in the system. These materials approach and reach a concentration at which they may cause severe pitting and corrosion which eventually requires replacement of the metal parts. Various corrosion inhibitors have been previously used.
Chromates and inorganic phosphates or polyphosphates have been used in the past to inhibit the corrosion of metals which is experienced when the metals are brought into contact with water. The chromates, though effective, are highly toxic and, consequently, present handling and disposal problems. Phosphates are nontoxic. However, due to the limited solubility of calcium phosphate, it is difficult to maintain adequate concentrations of phosphates in many instances. The polyphosphates are also relatively non-toxic, but tend to hydrolyze to form orthophosphate which in turn, like phosphate itself, can create scale and sludge problems in aqueous systems (e.g. by combining with calcium in the system to form calcium phosphate). Moreover, where there is concern over eutrophication of receiving waters, excess phosphate compounds can provide disposal problems as nutrient sources. Borates, nitrates and nitrites have also been used for corrosion inhibition. These too, can serve as nutrients in low concentrations, and/or represent potential health concerns at high concentrations.
In addition, environmental considerations have also recently increased concerns over the discharge of other metals, such as zinc, which previously were considered acceptable for water treatment.
Much recent research has been concerned with the development of organic scale and corrosion inhibitors which can reduce reliance on the traditional inorganic inhibitors. Among the organic inhibitors successfully employed are numerous organic phosphonates. These compounds may generally be used without detrimentally interfering with other conventional water treatment additives. There is a continuing need, however, for safe and effective water treatment agents which can be used to control corrosion.
Many of the organic scale inhibitors and corrosion inhibitors used in industrial aqueous systems (e.g. hydroxyethylidene diphosphonic acid) are themselves very sensitive to calcium hardness and prone to form deposits of their calcium salts. This limits the range of hardness in which such materials can be usefully applied as scale inhibitors or corrosion inhibitors. There is a continuing need for safe and effective water treating agents which can be used to control scale formation and to inhibit corrosion, particularly when substantial calcium carbonate is present in the system water.